This is a sound moderator aka "silencer" for an airgun or paintball gun. Pump-up airguns in particular make a lot of noise as the air rushes out, and the moderator cotains a series of what are called "K Baffles" (see cutaway one in the photo) that slow it down so that it does not make a sharp >Crak!< The pellet goes unimpeeded down the middle, and may even slightly gain velocity. This design is remarkably efficient.

Supersonic projectiles will always go crack anyway and a silencer is pretty pointless with them. Firearms are likely to blow the silencer to bits.

Instructions

The STL file supplied is to fit a .177"/4.5mm airgun with an 18mm external barrel diameter (Zoraki pistol). The SCAD file is parametric and allows the diameters, number of baffles and so forth to be varied to suit your requirement.

Print the silencer in quality ABS or Diamond Age Solutions' IMPLA as it has to hold a fair bit of pressure. Sacrifice build strength for beauty, and do not use support material.

WITH THE GUN UNLOADED and checked, fit the silencer and look down the bore to ensure things are central. Ideally, fasten the silencer in place with a hose clamp to avoid it becoming misaligned.

Finally, although silencers on airguns and paintball guns are generally legal you may wish to confirm the legality with a trusted authority